Apparatuses that convert three dimensional image data (volume data) such as 3DCT (Three Dimensional Computed Tomography) images into images suited for observation and display these images have been proposed as apparatuses for assisting image diagnosis by radiologists. Known conversion methods include: VR (Volume Rendering); MIP (Maximum Intensity Projection); and MPR (Multi Planar Reconstruction).
Images suited for observation differ according to observation targets and the purposes of observation. VE (Virtual Endoscopy) images are suited for observation of the inner walls of hollow tubular organs. In addition, images that represent tubular hollow organs which are linearly extended then cut open along the cores thereof (extension expanded images) and images that represent tubular hollow organs which are linearly extended then cut into cross sections perpendicular to the cores thereof (extension sectional images) are preferred, as they enable the entireties of tubular hollow organs to be observed in short amounts of time (refer to Patent Document 1).
However, virtual Endoscopy images, extension expanded images, and extension sectional images are images that capture images of the inner walls of tubular hollow organs from a single direction, and therefore there are many portions that do not appear in such images. For example, there are a great number of folds in the inner wall of the large intestine. However, if the direction of a line of sight which is set when an image is generated is perpendicular to the side wall of a fold, diseased portions (polyps, for example) which are present on the rear surface or behind the fold will not appear in the generated image. In addition, there are cases in which diagnosis is difficult even if diseased portions appear in images, if observation is only from a single direction. For example, fine recesses and protrusions of inner walls are difficult to discern by observation from directions perpendicular thereto, and there are cases in which polyps and recesses, and folds and grooves cannot be distinguished.
As a solution to this problem, Patent Document 2 (particularly, paragraph 0039) proposes a method in which the direction of a virtual light beam emitted from a virtual viewpoint is variable, and projection images from different images are displayed either alongside each other or as an animated series of images. In addition, Patent Document 3 proposes a method in which a display range movable in the direction of the core of a tubular hollow organ (the extension direction in an extension image), lines of sight are set such that they are perpendicular to an inner wall at regions corresponding to the center of the display range, and become smaller acute angles at regions further away from the center of the display range, and images that represent each of the regions are generated. In this method, the directions of set lines of sight will differ for the same location along the inner wall if the location is displayed as the center of the display range and displayed as the ends of the display range. Thereby, structures can be observed from different angles by changing the display range.